Abstract

Background There is huge individual variation in people's response
to negative life events.

Aims To test the hypothesis that genetic factors moderate
susceptibility to the environmentally mediated risks associated with negative
life events.

Method The Virginia Twin Study of Adolescent Behavioral Development
(VTSABD) was used to study the effects of independent life events (assessed
from maternal interview) on depression/anxiety (assessed from child interview)
in 184 same-gender female twin pairs, aged 14-17 years, measured on two
occasions.

Results There was no genetic effect on the independent negative life
events studied. A significant gene—environment interaction was found
using structural equation modelling. There was no effect of independent life
events on adolescents' depression in the absence of parental emotional
disorder, but a significant effect in its presence.

Conclusions There is an environmentally mediated effect of life
events on depression/anxiety. Genetic factors play a significant role in
individual differences in susceptibility to these environmentally mediated
risks.

There are substantial associations between negative life events carrying
long-term psychological threat and the onset of depressive disorder in adults
(Brown & Harris, 1978;
Kessler, 1997). The same may
apply in childhood and adolescence
(Goodyer, 1995). However, there
is huge individual variation in response. Although probably a majority of
depressive disorders are associated with severely negative life events or
experiences, only about one in five occurrences of severely negative life
events leads to depression (Brown et
al, 1987). A key question concerns the origins of this
individual variation in susceptibility to life events. Using an ingenious, but
indirect, means of inferring genetic risk, Kendler et al
(1995) showed that, in adults,
negative life events were most likely to lead to onset of major depressive
disorder in individuals inferred to have a genetic liability to depression. We
set out to undertake a more direct test of the hypothesis that genetic factors
moderate susceptibility to the environmentally mediated risks associated with
negative life events.

This has been as evident in longitudinal studies as in genetic
investigations. Thus, Champion et al
(1995) found that children who
showed emotional or behavioural disturbance on a teachers' questionnaire at
age 10 years were more than twice as likely as those without disturbance to
experience severely stressful events and experiences some 18 years later. The
finding is entirely consistent with what is known about stressful life events
and experiences (Rutter,
2001). The psychopathological risks are greatest in the case of
happenings that threaten important social relationships through loss,
humiliation, entrapment, hostility or conflict. Because social relationships
are dyadic and bidirectional, it follows that the ways in which one person
behaves are likely to shape other people's responses and, hence, to increase
or decrease the likelihood that the person will experience stressful life
events.

This led to the crucial differentiation made by Brown & Harris
(1978) between
‘
independent’ and ‘dependent’ life events. It was
appreciated from the outset that experiences people brought about through
their own behaviour could still provide a psychopathological risk (see
Rutter et al, 1993).
Nevertheless, in testing the hypothesis that stressful life events had a truly
causal impact on the development of psychopathology, it was methodologically
helpful to focus on those that, by their nature, were unlikely to have been
brought about through the person's own behaviour.

This methodological innovation, together with others, was vitally important
in establishing the case that life events were likely to play a causal role in
the onset of depressive disorders (Rutter,
2000). On their own, however, case—control designs are
inadequate for the study of nature—nurture interplay in the
susceptibility to environmental hazards. That is because, necessarily, the
difference between depression groups and control groups in life events will be
accompanied by parallel differences in both genetic liability and earlier
exposure to other environmental risks
(Robins & Robertson,
1998). Genetic designs are crucial in order to put the
environmental mediation hypothesis under rigorous test. The solution lies in a
focus on the association between life events and psychopathology within
monozygotic twin pairs, who do not differ in genetic liability; in treating
life events as a phenotype in bivariate analyses that use both cross-twin and
cross-trait associations; and in studying life events that involve no genetic
mediation (Kendler et al,
1999).

In twin designs, it is not possible to identify gene—environment
interactions (G × E) in an unambiguous manner if there
are also gene—environment correlations (rGE). Consequently, if
nature—nurture interplay is to be analysed in its various facets, it
will be necessary to adopt different strategies for the investigation of
G × E (genetically influenced sensitivity to the
environment) and rGE (an association between genetic and
environmental risks).

In order to study G × E, we examined life events
with a demonstrated empirical association with depression/anxiety within our
own data-set (i.e. those for which there was a prima facie case for possible
environmental mediation of risk), focusing on samples in which there was also
empirical evidence of genetic liability for depression/anxiety (this meant
restriction to adolescent girls). The next step was to subdivide these events
into those that, by their nature, seemed dependent and those that seemed
independent. The rationale was that rGE was likely to be operative in
the former and absent in the latter, although that assumption had to be
tested. The former provided the opportunity to study the role of rGE
in the liability to depression/anxiety. The findings showed that dependent
life events were significantly associated with depression within monozygotic
twin pairs (Silberg et al,
1999; Rutter,
2000); therefore, despite being brought about by the person's own
behaviour, the experience exerted true environmentally mediated risk for
depression from life events.

The next stage was to focus on independent life events (for which
rGE was unlikely) in order to test for G × E.
The postulate was that the depression/anxiety would be more likely to be
associated with negative life events in individuals with a genetic liability.
The research strategy involved four interconnected steps. First, it was
necessary to establish that there was a significant effect of life events on
depression/anxiety and that there was no appreciable genetic effect on such
life events. Second, we tested for a significant G × E
interaction such that the genetic effect was greater in the presence of life
events, indicating that an individual's inherited liability to symptoms of
depression (i.e. sensitivity) is affected by exposure to certain environments.
Although it sounds paradoxical, that implies genetic moderation of sensitivity
to life events. That is because, according to standard practice, all effects
of G × E are included in the genetic term.
Accordingly, the third step was to split the genetic term into the effects of
baseline genes and the effect of G × E. If the genetic
moderation of environmental risk mediation was correct, the whole of the
difference in genetic effect according to the presence or absence of life
events should be accounted for by G × E. The fourth
step involved testing whether the effect of life events was greater in the
presence of parental emotional disorder. Clearly, parental disorder will index
more than just genetic risk, but if the genetic moderation hypothesis was
correct it should follow that the effect of life events would be greater in
the presence of parental emotional disorder than in its absence.

METHOD

The analysis was based upon data collected on girls aged 14 years or more
from the Virginia Twin Study of Adolescent Behavioral Development (VTSABD).
Details concerning the design, ascertainment, assessment protocol and
participation rates for this study are provided elsewhere
(Hewitt et al, 1997;
Simonoff et al,
1997). Because our earlier findings had shown that genetic effects
on depression were greater in pubertal adolescents than in children and
greater in females than in males, we focused on girls who were likely to be
well advanced in the transition through puberty, i.e. at least 14 years old
(Silberg et al,
1999). Our analyses had also shown that this was an age period
when life events were likely to be having an effect on depression and anxiety.
There were 184 same-gender female pairs, with two waves of data collection
some 15 months apart.

Assessment of depression and anxiety

Symptoms of depression and generalised anxiety disorder were assessed
according to their presence in the 3 months immediately preceding interview,
using the child's responses to the Child and Adolescent Psychiatric Assessment
(CAPA; Angold et al,
1995). This is an investigator-based psychiatric interview that
provides information for diagnosing the major forms of childhood
psychopathology according to DSM—IV criteria
(American Psychiatric Association,
1994). Each symptom diagnostic of either a major depressive
episode or generalised anxiety disorder was coded as if the symptom were
present in at least two areas of activity in the child's life and at least
somewhat uncontrollable. Otherwise it was considered not to be present. The
respective symptoms were then summed into a depression and anxiety sub-scale
for subsequent data analysis.

Selection of life events

A life events scale was composed from the maternal ratings of events in the
preceding year (Johnson & McCutcheon,
1990) that were significantly associated with depression, but were
considered to be beyond the child's control (independent life events).
Dependent life events (events potentially within the child's control) have
been shown to be genetically correlated with risk to depression in this sample
(Silberg et al,
1999). Theoretically, if particular life events are linked to a
common set of genes that also influence risk to depression (a genotype ×
environment correlation), there could be increasing genetic variance for
depression in the presence of those same events. Because the inclusion of
events that are dependent on the child's genotype can potentially give rise to
the same pattern of results as a gene—environment interaction, for
clarity of interpretation only independent life events were included in the
G × E interaction analysis.

Since a G × E interaction is considered to be
operating only if there are increasing genetic differences as a function of
differences in exposure to a true environmental risk factor, the twin
correlations for the independent life events were also estimated in
monozygotic (MZ) and dizygotic (DZ) post-pubertal females using SAS
(SAS Institute, 1996).

Epidemiological analyses

The PROC MIXED procedure in SAS was used to select life events
significantly related to symptoms of depression. The procedure takes account
of the clustering of observations from twins of a pair and across the two
waves of data as part of the fitted model. The PROC LOGISTIC procedure was
then used to estimate the odds ratio of life events to symptoms of depression
and anxiety using the top 10% of scores as a cut-off for generating a
categorical index of the two traits.

Model fitting

Raw data collected from the female twin pairs from the first and second
waves of data were analysed by maximum likelihood using Mx 1.47 (details
available from M.N. upon request). The basic genetic model comprises additive
genetic (A), common (C) and specific (E) environment components of variation.
In the G × E interaction model (diagrams of the model
are available from the author upon request), we examine whether the genetic
factors contribute the same amount to the variance under different
environmental conditions, in this case the measured level of independent life
events. We use a multi-level extension of this method
(Neale & Cardon, 1992) by
specifying a different model according to the level of stress assessed in each
twin. The covariance between depression and anxiety is modelled as one factor
that influences both traits, and a second factor that influences anxiety only.
The genetic part of the model is subdivided into two components: a
‘
baseline’ component and a component that varies according to the
stress level. By comparing the fit of the model with the moderating effect of
stress with that of a model without moderating effects, we can judge the
statistical significance of the moderation (interaction) using a likelihood
ratio test. The difference between —2(log likelihood) of the two models
is distributed as a chisquare (χ2), and degrees of freedom
(d.f.) are calculated by subtracting the number of estimated parameters of a
reduced model from those estimated in the fuller model. To avoid a
false-positive finding when testing for a moderating effect of the
environment, the main effect of life events is regressed out of both the
depression and anxiety sub-scales just prior to model fitting.

A critical test of the G × E interaction model is
that the data cannot be explained by stress-related differences in the
specific environment. This could arise from heteroscedasticity, or increasing
error variance in groups of individuals with higher mean levels of stress. A
model that allowed the unique environmental parameter to differ in the various
exposed groups was compared with the fit of the G × E
interaction model that allowed for differences in genetic variance as a
function of environmental exposure. To assess the moderating effect of life
events on the shared environmental variance, a model that allowed this
parameter to vary across the different life event groups was also fitted to
the data on depression and anxiety and similarly evaluated against the
G × E interaction model.

As another test of a G × E interaction, maternal
and paternal history of generalised anxiety disorder or major depressive
disorder (assessed on the basis of systematic interview data) was used as an
index of familial risk for depression and anxiety in the child. Although
parental psychopathology could represent a genetic or shared environmental
liability (or more probably a combination of the two), if a G ×
E interaction were operating, we would expect an increased risk for
depression or anxiety as a function of the presence of life events and
psychiatric disorder in one or both parents. The PROC MIXED procedure was used
to test for mean differences in depression and anxiety as a function of life
events in those with and without a history of parental anxiety/depression.

RESULTS

Three past-year events reported by the girls' mothers were significantly
related to child-reported depression. These were:

a new stepbrother or stepsister;

brother or sister (or stepbrother/stepsister) leaving home;

the father losing his job.

Taken together, these three events had a correlation of 0.11 with
depressive symptoms and 0.06 with anxiety symptoms, comparable to odds ratios
of 1.5 and 1.3, using a dichotomised measure of the two traits. This
phenotypic association indicated that life events had an effect on emotional
difficulties in adolescent girls, statistically significant (P=0.001)
in the case of depression and just short of significance for anxiety
(P=0.07). There was a need to accompany this demonstration with a
test of whether it was likely that genetic mediation could be involved. All
three events were of a kind that should be shared across twins within the same
pair and hence very high within-pair correlations were to be expected. The
question was whether, despite sharing, there was a difference between MZ and
DZ pairs. The findings showed that there was not. The average correlation for
the life events sub-scale across the two waves was 0.91 for MZ twins and 0.87
for DZ twins, evidence that these life events are not dependent upon the
genotype of the child, but were truly environmental in nature.

Having shown that the life events had a significant effect on
depression/anxiety and that this represented environmental mediation, the next
step was to test for G × E interaction. A model that
allowed for increasing genetic variance as a function of environmental
exposure fitted relatively better than a model that allowed for increasing
unique environmental variance or shared environmental variance as a function
of number of life events (further details available from the author upon
request). The model fitting results shown in
Table 1 indicate that
eliminating the interaction term from the full G × E
interaction model (Model II) resulted in a significant deterioration in fit
for both depression (Model I) and anxiety (Model III). These results also
showed that exposure to life events increases the genetic association or
genetic covariance between the two traits (Model IV).

The better fit of the G × E interaction model
indicates that there are genetically mediated differences in adolescent girls'
sensitivity to the effects of the environment. The proportions of overall
variance in depression and anxiety due to genetic effects for each level of
life event from this model are depicted in
Fig. 1. For depression, the
genetic variance rose from 0.27 in the absence of life events, to 0.33 for one
life event and to 0.39 for two life events. The comparable estimates for
anxiety were 0.19, 0.34 and 0.44 respectively. Although there was a trend for
greater moderation of genetic variance for anxiety in the presence of one or
more life events, it is not significantly different from the increase in the
environmentally influenced change in genetic variance observed for
depression.

Change in genetic variance for depression (solid line) and anxiety (dashed
line) as a function of number of past-year life events.

The third step in the analysis involved determination of whether the
difference in the genetic term according to the presence of life events was
true owing to G × E. The components of variance for
depression and anxiety attributable to baseline genes and G ×
E interaction under different levels of events are shown in Figs
2 and
3, which demonstrate that all
of the change in genetic variance for depression and anxiety in the presence
of one, two or more life events was due to the effect of the interaction
between genes and environment.

Effect of life events on variance in anxiety symptoms accounted for by
‘
baseline’ genetic effects (triangles) and by
gene—environment interaction (squares).

The fourth and final step involved testing of the effects of life events in
the presence and absence of parental emotional disorder.
Table 2 presents the average
CAPA depression and anxiety scores (standardised to a mean of 10 with a
standard deviation of 3) contingent on exposure to life events and parental
psychopathology, an index of familial risk. The first finding was that there
was no significant — or even appreciable — effect of life events
when there was no emotional disorder in either parent (9.57 v. 9.56,
P=0.98; 9.52 v. 9.60, P=0.85). The strong
implication is that for life events to have any causal effect on
depression/anxiety in adolescent girls there must be either a genetic
predisposition or an increased vulnerability resulting from the environmental
effects of parental emotional disorder. Without these, there was no
demonstrable psychopathological risk associated with the life events studied.
The second finding was a significant effect of life events on depression
(10.05 v. 10.98; P=0.01) when these occurred against the
background of parental emotional disorder (the trend for anxiety was similar
but fell well short of significance, P=0.15). This suggests that,
given genetic or environmental vulnerability, life events do have an important
environmental role in the causation of depressive symptomatology. The third
finding was that parental emotional disorder had an effect on
depression/anxiety both in the absence of life events (9.57 v. 10.05,
P=0.04 for depression; 9.52 v. 10.16, P=0.007 for
anxiety) and in its presence (9.56 v. 10.98, P=0.02 for
depression; 9.60 v. 10.64, P=0.04 for anxiety). The
implication is that part of the genetic mediation involves G ×
E but much does not (at least in so far as the life events indexed
environmental risk). The pattern of results for depression is consistent with
a G × E interaction — the girls with the highest
depression scores are those who experienced a life event in the past year and
who had a mother or father (or both) with a history of depression or
anxiety.

Level of depressive and anxiety symptoms in adolescent girls according
to parental emotional disorder and presence of one (or more) life
events

DISCUSSION

Despite the remarkable degree of individual variation in response to
psychosocial experiences, surprisingly little is known about the causal
mechanisms underlying this variation. This applies as much to the effects of
negative life events on liability to depression, as to the other experiences.
In order to determine why people vary in their susceptibility to environmental
stressors, it is crucial to start with a research strategy that can provide a
rigorous test of environmental mediation of psychopathological risk. Until
such mediation is established, it is not possible to be sure that
susceptibility to psychosocial risks is truly being examined. That has posed a
substantial limitation in view of the evidence that there are important
gene—environment correlations
(Plomin, 1995) and, hence,
that the risks associated with negative events and experiences could be
genetically mediated in part. To rule out genetic mediation, it is desirable
to use genetically sensitive designs.

In studying the effects of negative life events on depressive and anxious
symptomatology in adolescent girls, we used a twin design that allowed us to
use two different tests of environmental mediation. First, focusing on
child-specific events, we had shown previously that life events were
significantly associated with depression even within MZ pairs, who shared the
same genetic risks (Silberg et
al, 1999; Rutter,
2000). Second, by focusing on life events that impinged equally on
both twins, we confirmed that there was no significant within-pair genetic
effect on such life events, and that the significant effect of life events on
depression/anxiety had therefore to be environmentally mediated.

In keeping with other studies, we found major individual variation in
susceptibility to life events. Only a minority of the adolescent girls
experiencing life events showed depression/anxiety. We tackled the hypothesis
that part of the individual variation in susceptibility could be genetically
influenced by testing for G × E interaction.
Significant interaction was found. Genetic effects on depression/anxiety, and
especially on their co-occurrence, were substantially and significantly
greater in individuals who had experienced negative life events in the past
year than in those without such experiences. We went on to show that this
difference in genetic effect was due to G × E
interaction, there being no difference in the effects of baseline genes.

Finally, we employed a quite different test of interaction by comparing the
effects of life events on depression/anxiety in the presence and in the
absence of parental emotional disorder. The findings showed no effect of life
events in girls whose parents had not experienced emotional disorder, a result
that is entirely consistent with the hypothesis that G ×
E interaction accounts for much of the individual variation in
susceptibility to life events. Equally, we found that the effects of parental
emotional disorder were significantly greater in the presence of life events
than in their absence, again pointing to the likely role of G ×
E interaction. On the other hand, there was a significant effect of
parental mental disorder in the absence of life events, indicating that life
events did not constitute the only route of risk mediation.

In order to test for G × E interaction, it was
necessary for methodological reasons to focus on a subset of life events that
did not show a gene—environment correlation. Almost certainly that
resulted in our studying life events with a relatively weak risk effect on
depression/anxiety. In order to provide a better estimate of the overall
environmentally mediated risk for depression/anxiety associated with life
events, it will be necessary to use bivariate analyses, treating total life
events exposure as a phenotype. That will constitute the next step in our
overall strategy to study the effects of life events. However, that method
will not allow any satisfactory investigation of G × E
interaction (the focus of this study).

The findings do not, of course, mean that the whole of individual variation
in susceptibility to environmental risks is due to G ×
E interaction. It is highly likely that both previous environmental
risk exposure and concurrent exposure to other more long-standing psychosocial
adversities also play a role in individual variations in susceptibility. The
investigation of that possibility will require the use of different designs.
Also, greater leverage on the operation of environmental risks can be obtained
by using the three waves of data collection (spanning some 3 years) available
in VTSABD and by putting together a greater range of psychosocial risk
factors.

Our study of the effects of life events had to rely on a questionnaire
measure, which is inevitably cruder than the detailed investigator-based
methods that have come to be accepted as the optimal approach. The consequence
is that the true effect of life events on depression/anxiety may well be
greater than that found here. On the other hand, our reliance on a
questionnaire measure is unlikely to have biased our finding of a significant
G × E interaction. We took particular care to use
methods that avoided the possibilities of criterion contamination deriving
from reliance on a single informant for both independent and dependent
variables — a major limitation of most life events studies
(Rutter, 2001). Thus, we used
parental reports for life events and child reports for depression/anxiety. A
further problem in much life events research has been the possibility that the
life events may have been caused by the person's behaviour, rather than the
other way round. Both our selection of life events and our use of a twin
design effectively ruled out that possibility. Our data were cross-sectional
rather than longitudinal and, hence, we could not focus specifically on the
role of life events in provoking the onset of disorder. However, that was not
our goal. Rather, our focus was on the liability to depression/anxiety, for
which the methods used were well suited. Nevertheless, longitudinal data, to
which we will turn next, will help in sorting out causal processes because
they will provide a better leverage on recurrent/chronic symptomatology and
because they will allow us to examine the extent to which the
psychopathological risks associated with life events derive from their
accumulation over time rather than their occurrence during a specified period
prior to the onset of a new disorder.

A further query concerns the nature of the phenotypic individual feature
that indexes susceptibility to environmental stressors. Psychosocial
researchers have tended to favour explanations in terms of cognitive sets
(Brown & Harris, 1990)
whereas behavioural geneticists (Kendler
et al, 1999) and others
(Andrews, 1996) have usually
focused on the personality trait of neuroticism. These (and other)
alternatives need to be put to the test, not just in relation to a general
liability to depression/anxiety but specifically in relation to their possible
role as influences on susceptibility to life stressors.

Meanwhile, the message from our findings is that genetic factors have a
significant role in the individual response to environmental stressors. It is
also evident that genetic effects on liability to depression/anxiety must be
considered as operating through indirect, as well as direct, routes. That is,
they operate in part because they influence both exposure to environmental
risks (gene—environment correlations) and susceptibility to the
psychopathological effects of risk environments (gene—environment
interactions). Traditional forms of analysis attribute the whole of both these
indirect effects to genes, but it is apparent that this is misleading. The
psychopathological risks come about as a specific result of the conjunction,
and joint operation, of genetic and environmental influences. The task of
delineating the causal mechanisms underlying the interplay between nature and
nurture constitutes a major research priority for the future. The well-known
finding of increasing depression in adolescent girls may be due to increasing
genetic differences in sensitivity or reactivity to the environment.

Clinical Implications and Limitations

CLINICAL IMPLICATIONS

Negative life events have an environmentally mediated effect on
depressive/anxious symptomatology.

Individual differences in susceptibility to stressful life events are
due, in part, to genetic influences.

Psychosocial risks for depression/anxiety are most likely to be
operative in individuals who are also at genetic risk.

LIMITATIONS

Life events were measured by questionnaire rather than
interview.

For unavoidable methodological reasons it was necessary to focus on a
subset of independent life events.

Analyses were based on cross-sectional, rather than longitudinal,
associations.